System and method of controlling the delivery of medical gases to a patient

ABSTRACT

A system for controlling the delivery of medical gases includes a digital signal processor that receives at least one ventilation parameter value change, calculates a fresh oxygen flow rate, total fresh gas flow rate into the breathing circuit, and a reference oxygen flow rate representative of a predetermined oxygen concentration delivered to a patient. A graphical display presents the calculated fresh oxygen flow rate, total fresh gas flow rate, and reference oxygen flow rate. A method of controlling the delivery of medical gases to a patient includes calculating a total fresh gas flow rate into the breathing circuit, calculating a fresh oxygen flow rate into the breathing circuit, calculating a reference oxygen flow rate representative of a predetermined oxygen concentration delivered to the patient and presenting the total fresh gas flow rate, the fresh oxygen flow rate, and reference oxygen flow rate on a graphical display.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/987,544, filed on Jan. 10, 2011.

BACKGROUND

The present disclosure is related to the field of mechanicalventilation. More specifically, the current disclosure is related to theprevention of the delivery of hypoxic gases to a patient.

Known guards against the delivery of hypoxic gases are based uponminimum concentration settings to ensure the addition of adequate freshgas oxygen to a breathing circuit. Such a minimum concentration settingis maintained with a mechanical or equivalent link between the gas flowsettings of the constituent gases of the delivered fresh gas. Suchlinkages ensure that the resulting mixture of fresh gas maintains arequired minimum oxygen concentration.

During some mechanical ventilator operational settings, particularly lowflow mechanical ventilator operation, hypoxic gases can still bedelivered to a patient, even when known hypoxic guards register that theminimum oxygen concentration for fresh gas flows has been met.

BRIEF DISCLOSURE

A system for controlling the delivery of medical gases duringrespiratory support of a patient includes a breathing circuit with apatient connection configured to deliver respiratory support to thepatient. A mixer is pneumatically connected to the breathing circuit.The mixer is configured to provide at least oxygen and a balance gas tothe breathing circuit. A mechanical ventilator is pneumaticallyconnected to the breathing circuit and is configured to create fluidpressure waveforms within the breathing circuit to provide respiratorysupport to the patient. A digital signal processor is communicativelyconnected to the mixer and the mechanical ventilator. The digital signalprocessor receives at least one ventilation parameter value change,calculates a fresh oxygen flow rate into the breathing circuit and totalfresh gas flow rate into the breathing circuit with the at least oneventilation parameter value. The digital signal processor calculates areference oxygen flow rate representative of a predetermined oxygenconcentration delivered to the patient with at least the total fresh gasflow rate. A graphical display is communicatively connected to thedigital signal processor. The graphical display presents the calculatedfresh oxygen flow rate, the total fresh gas flow rate, and the referenceoxygen flow rate.

A low-flow ventilation system provides repeating breaths of respiratorysupport to a patient. The low-flow ventilation system includes abreathing circuit with a patient connection configured to deliver therespiratory support to the patient. An input device is operable by aclinician to input at least one ventilation parameter value. A mixer ispneumatically connected to the breathing circuit and is configured toprovide at least oxygen and a balance gas to the breathing circuit. Avaporizer is pneumatically connected to the breathing circuit and to themixer. The vaporizer receives at least the oxygen and the balance gasfrom the mixer and entrains anesthetic agent vapor into the receivedoxygen and balance gas. A digital signal processor is communicativelyconnected to the input device. The digital signal processor receives theat least one ventilation parameter value, calculates a fresh oxygen flowrate into the breathing circuit, calculates a total fresh gas flow rateinto the breathing circuit, calculates a predicted oxygen concentrationdelivered to the patient, and compares the predicted oxygenconcentration to a predetermined minimum oxygen requirement threshold ofthe patient. A graphical display is communicatively connected to thedigital signal processor and presents the calculated fresh oxygen flowrate and total fresh gas flow rate. If the predicted oxygenconcentration is above the predetermined minimum oxygen requirementthreshold, the digital signal processor accepts the at least oneventilation parameter value.

A method of controlling the delivery of medical gases to a patientreceiving ventilatory support from a low-flow mechanical ventilatorincludes providing respiratory support to the patient through abreathing circuit pneumatically connected to a mechanical ventilator ina low-flow configuration. A flow of medical gas comprising oxygen isreceived with a mixer pneumatically connected to the breathing circuit.A ventilation parameter value change is received with a digital signalprocessor. A total fresh gas flow rate into the breathing circuit iscalculated with the digital signal processor from the receivedventilation parameter value change. A fresh oxygen flow rate into thebreathing circuit is calculated with the digital signal processor fromthe received ventilation parameter value change. A reference oxygen flowrate is calculated with the digital signal processor. The referenceoxygen flow rate achieves a predetermined oxygen concentration asdelivered to the patient based upon the total fresh gas flow rate. Thetotal fresh gas flow rate, the fresh oxygen flow rate and referenceoxygen flow rate are presented on a graphical display.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an embodiment of a low-flow ventilationsystem.

FIG. 2 is a flow chart that depicts an embodiment of a method ofcontrolling the delivery of medical gases to a patient.

FIG. 3 depicts an example of a screen shot of an embodiment of agraphical user interface.

FIG. 4 is a flow chart that depicts an embodiment of a method ofcontrolling the delivery of medical gases.

DETAILED DISCLOSURE

FIG. 1 is a schematic diagram of a system 10 for providing respiratorysupport to a patient 12 while preventing the delivery of hypoxic gasesto the patient 12.

A respiratory support system 10 includes a breathing circuit 14 throughwhich inspiratory gases 16 are provided to the patient and expired gases18 are directed from the patient 12. Check valves 20 within thebreathing circuit 14 ensure the unidirectional flow of gas within thebreathing circuit 14.

In a high flow respiratory support system most or all of the expiredgases 18 are vented, exemplarily through relief valve 26, to the ambientair.

In low flow respiratory support system, gases are added and removed fromthe breathing circuit 14. Attributed to metabolism, the patient 12 addscarbon dioxide and removes oxygen from the breathing circuit 14. Duringexpiration, the expired gases 18 are directed to an absorber 24 wherethe carbon dioxide produced by the patient 12 is removed. Some smallamount of gas 28 (less than 0.3 liters per min) is leaked from thebreathing circuit 14. The remaining expired gas volume is stored in theventilator 40 to be kept in reserve for the next tidal breath. To makeup for the loss of gases by the patient 12 and leaks 28, fresh gas 46 isadded to the breathing circuit. Typically the fresh gas 46 is added(about 0.5 to 3 liters per min) in excess of the total gas loss andforces the removal of some of the expired gas 18 through a relief valve26. Concentration of gases from the relief valve 26 have approximatelythe same concentration as the patient expired gases. The ventilator 40delivers the next tidal breath by compressing the remaining reservevolume. The unidirectional check valves 20 control the direction of gasflow in the breathing circuit 14 for the inspiratory gases 16 going tothe patient 12 and expired gases 18 from the patient 12.

In this embodiment, the fresh gas 46 comes from the fresh gas manifold34. The fresh gas 46 comprises at least two medical gases from a mixer70 and/or an anesthetic vapor from an anesthetic vaporizer 32. The mixer70 receives flows of two or more medical gases. The flow rate of theconstituent medical gases into the mixer are controlled to create acombined medical gas with a set concentration of constituent medicalgases. Often a combination of individual gas (for example oxygen)concentration and total flow rate or individual gas flow rates are usedto control the flow of medical gas into the mixer. These mixer controlsettings can also be interchangeably calculated and used.

Fresh gas from the mixer 70 is provided through conduit 72 to theanesthetic vaporizer 32. In the vaporizer 32, a portion of the gas isdiverted through a sump (not depicted) of the vaporizer 32 to serve as acarrier gas to pick up an amount of saturated anesthetic vapor accordingto the concentration settings of the vaporizer 32. All the gases fromthe mixer 70 and the anesthetic vapor from the vaporizer 32 arepneumatically conducted as fresh gas 46 via conduit 74 to the breathingcircuit 14.

In another embodiment, the anesthetic vaporizer 32 directly injects therequired anesthetic vapor into the breathing circuit 14 via conduit 74.Since no carrier gas is required to pick up the anesthetic vapor, thegases from mixer 70 is added directly to the breathing circuit 14 byrerouting the outlet of conduit 72 from the anesthetic vaporizer 32 tothe breathing circuit 14 (not shown). Conceptually the replacement freshgas 46 from the fresh gas manifold 34 consists all of the gases andvapor in the conduits 72 and 74.

A normal range of oxygen concentration inspired by the patient 12 isbetween 21% and 30%. A normal range of an oxygen concentration in gasesexpired by the patient 12 is between 17% and 25%.

Therefore, under normal conditions, the expired breathing gases maycontain an oxygen concentration of 17% which is only 80% of the minimumrequired oxygen concentration (21%) to be recirculated to the patient.In high fresh gas flow respiratory support systems, this is not aproblem as the fresh gas is held at a minimum oxygen concentration of21% with current hypoxic guards. However, in a low-flow system, eventhough the gas loss 26, 28 in the respiratory support system is smalland this requires only a small amount of replacement fresh gas, theeffect of this gas loss on the required composition of the fresh gas issignificant. Under these conditions, even though the minimum gasconcentration level (e.g. 21%) at the fresh gas manifold 34 is met, thesmall volume of fresh gas is insufficient to raise the oxygenconcentration of the inspired gases 16 delivered to the patient to theminimum oxygen concentration (e.g. 21%) required by the patient.

Additionally, the patient 12 may require a greater concentration ofoxygen due to the physiological condition of the patient 12. In theseinstances, while the patient 12 may be delivered a technicallynon-hypoxic gas mixture for the normal population, the inspired gasesdelivered to the patient 12 may be effectively hypoxic for the oxygenconcentration requirements of that specific patient 12.

Therefore, the presently disclosed systems and methods control theprovision of fresh gas through the fresh gas manifold 34 in order toprevent the delivery of hypoxic gases to the patient 12.

A digital signal processor (DSP) 36 is communicatively connected to thefresh gas manifold 34 and to the anesthetic vaporizer 32, if one isused. The DSP 36 is further communicatively connected to a computerreadable medium 38 that is programmed with computer readable code thatupon execution by the DSP 36 causes the DSP 36 to operate in the mannerdescribed herein and perform the functions as described herein. Thecomputer readable medium 38 may be any of a variety of configurations ofnon-volatile memory. In one embodiment, the computer readable medium 38is an integral part of the DSP 36. In an alternative embodiment, thecomputer readable medium is a separate component that is communicativelyconnected to the DSP 36. In one non-limiting embodiment, the computerreadable medium 38 is flash memory.

The DSP 36 is connected to a mechanical ventilator 40. The mechanicalventilator 40 is operated by the DSP 36 in order to provide repeatingwaveforms or breaths of inspired gas 16 to the patient 12. Themechanical ventilator 40 is operated by the DSP 36 to provide a varietyof forms of respiratory support, including full ventilation orspontaneous breathing assistance.

A respiratory gas monitor 42 is disposed in or near conduit 60 that isthe common y-piece of the breathing circuit 14. The respiratory gasmonitor 42 analyzes at least the concentration of the inspired gases 16flowing into the patient 12 and the expired gases 18 flowing away fromthe patient 12. The respiratory gas monitor 42 provides a signal 44 thatis indicative of the gas concentrations, particularly the oxygenconcentration. In still further embodiments, the respiratory gas monitor42 can provide a signal indicative of inspired and expired anestheticgas concentrations. The respiratory gas monitor 42 further includes aflow sensor such that the DSP 36 is provided with an indication of theflow of gas to and from the patient 12. The DSP 36 receives themeasurements of concentration and flow of gas breathed by the patient 12and uses this information in determining the amount of fresh gas 46 tobe introduced to the breathing circuit 14 from the fresh gas manifold34.

In an alternative embodiment, gas monitors supplemental to, or inreplace of, the respiratory gas monitor 42 are used within the breathingcircuit 14. Non limiting examples of such gas monitors include aninspiratory gas monitor (not depicted) disposed within the inspiratorylimb, or an expiratory gas monitor (not depicted) disposed within theexpiratory limb. In addition to these exemplary sensor configurations,alternative suitable configurations would be recognized by a person ofordinary skill in the art.

The mixer 70 of the fresh gas manifold 34 is connected to one or moresources of medical gas, such as oxygen, nitrous oxide, medical air,heliox, xenon, helium. In practice, the fresh gas manifold 34 is atleast connected to an oxygen source 48 and a balance gas source 50. Inan alternative embodiment, the fresh gas manifold 34 is connected to anair source. As atmospheric air approximately comprises 21% oxygen and78% nitrogen, this is a common source of medical gas used with a freshgas manifold 34 as the concentration of oxygen in the atmospheric airgenerally meets the minimum hypoxic guard concentrations as discussedabove.

An example will highlight differences between previous systems and thesystems and methods as disclosed herein. If a normal minute volume ofrespiratory support to a patient 12 is five liters per minute, then at aminimum oxygen concentration of 21%, the minute volume of oxygenprovided to the patient is 1.05 liters per minute. If the gas expired bythe patient with metabolic oxygen consumption of 0.2 liters per minutehas an oxygen concentration of 17% and 1 liter of gas is lost throughthe recirculation of the expired gases, then a 17% concentration ofoxygen of the 4 liters per minute of recirculated gases will contribute0.68 liters per minute to the rebreathed inspired gases. Therefore, if 1liter of fresh gas at 25% oxygen concentration is provided by the freshgas manifold 34 to replace the lost volume of breathing gases, theminute volume of oxygen in the combined fresh gas and recirculated gasprovided as inspiratory gases 16 will only be 0.93 liters per minute or18.6% oxygen concentration which would be considered a hypoxic mixtureand is insufficient to replenish the inspired gas concentration thatstarted at 21%. This shows how the known hypoxic guards at a minimumconcentration of 25% can allow hypoxic gas concentrations of less than21% oxygen to be delivered to the patient.

Rather, in the system 10 disclosed herein, the DSP 36 operates the mixer70 and fresh gas manifold 34 in a manner such as to control theconcentration and flow of the fresh gas 46 provided to the breathingcircuit 14, based upon the metabolic needs of the patient 12.

The DSP 36 uses a variety of equations in calculating thecharacteristics of the fresh gas 46 provided to the breathing circuit14. Equation (1) generally describes the relationship between the freshgas 46 delivered from the fresh gas manifold 34 and the oxygenconcentration requirements of the patient 12 and can be derived based onthe conservation of mass at a steady temperature and pressure.FGO₂F={dot over (V)}O₂+FeO₂(FGF−{dot over (V)}O₂  (1)

In the above equation, the fresh oxygen flow, FGO₂F, is the oxygen flowrate in the fresh gas. {dot over (V)} O₂ is the oxygen uptake rate ofthe patient. FeO₂ is the fractional expired oxygen, which is otherwiseknown as the concentration of oxygen in the expired gases 18. FGF is thetotal fresh gas flow rate, or the flow rate of the fresh gas 46 providedout of the fresh gas manifold 34. This equation assumes that all the CO2produced by the patient is completely removed by the carbon dioxideabsorber 24. A small correction factor using the fraction of the expiredcarbon dioxide (typically less than 5 or 6%) can be included to correctthe second term in equation 1 to account for the carbon dioxide lossthrough the relief valve 28 instead of preferentially absorbed by thecarbon dioxide absorber 24. The exposure of carbon dioxide to theabsorber 24 varies with the location of the absorber 24 and the excessgas relief valve 26. It is also known that if the fresh gas 46 isintroduced between the inspiratory unidirection valve and the patient(not shown), the gases lost through the relief valve 26 will includeinspiratory gases and more oxygen from the fresh gas is required toensure non-hypoxic gases delivered to the patient.

In embodiments that do not include a vaporizer, the FGF will be just asdescribed above. However, in embodiments that include a vaporizer 32 inthe fresh gas manifold 34, the FGF also includes the anesthetic agentvapor flow (AVF). The vaporizer 32 operates to entrain anesthetic agentvapor into the flow of oxygen and balance gas provided through thevaporizer. In a system that uses a mechanical vaporizer, the AVF is anestimate computed using a measurement of the fraction of agent inspired(FiA) as may be measured by a gas monitor disposed within the breathingcircuit. The AVF is calculated using the following equation:

$\begin{matrix}{{{AVF}\mspace{14mu}\left( {L\text{/}\min} \right)} = \frac{{MGF} \times {FiA}}{1 - {FiA}}} & (2)\end{matrix}$

In the above equation, the MGF is the total gas flow provided from themixer, therefore MGF can also be represented by the equation:MGF=BGF+FGO₂F  (3)

While equation (1) is written with respect to the fractional expiredoxygen, in consideration of the conservation of oxygen breathed over apatient breath, the fractional expired oxygen (FeO₂) is related tofractional inspired oxygen (FiO₂) by the following equation:

$\begin{matrix}{{FiO}_{2} = {{FeO}_{2} + \frac{\overset{.}{V}O_{2}}{MV}}} & (4)\end{matrix}$

In equation (2), FiO₂ is the fractional inspired oxygen, or theconcentration of oxygen in the inspiratory gases 16 provided to thepatient 12. MV is the minute volume of total gases that are provided tothe patient 12 by the respiratory support system 10. The minute volumemay be set by a clinician by entering the minute volume into an inputdevice 54. The minute volume is provided by the input device 54 to theDSP 36, and the DSP 36 operates the ventilator 40 to provide respiratorysupport to the patient 12 that achieves the established minute volume.The MV can also be measured using the respiratory gas monitor 42. Therespiratory gas monitor 42 communicates with DSP 36 via connection 44.

One or more ventilation parameter values of the system 10 are controlledby the clinician, such as by inputs into the input device 54. The DSP 36manages these input ventilation parameter values using equation (1) suchthat the clinician does not unknowingly establish a combination ofvalues that will result in a hypoxic gas mixture being delivered to thepatient 12. In an exemplary embodiment, one or more of the fresh gasflow rate (FGF), the fractional fresh gas oxygen (FFGO₂), and the freshgas oxygen flow rate (FGO₂F) are ventilation parameter values that canbe controlled by the clinician.

Additionally, the patient oxygen uptake rate (VO₂) can be obtained inseveral ways. The oxygen uptake can be measured from the patient bycomparing the volume of oxygen inspired and expired by the patient overa breath. In particular, the oxygen uptake is the integral of theinstantaneous product of the oxygen concentration and gas flow breathedby the patient over a breath. This oxygen concentration and gas flow ismeasured by the respiratory gas monitor 42. With the availability ofthese measurements, some respiratory gas monitors perform the oxygenuptake computation. Alternatively, these measurements can be fed via thesignal 44 to the DSP 36 to compute the integration. An estimation of theoxygen uptake can be obtained by multiplying the difference between FiO₂and FeO₂ by the MV. In an embodiment, the respiratory gas monitor 42measures the FiO2, FeO2 and MV.

In another embodiment, the oxygen uptake rate may be entered by aclinician through the input device 54 as a clinical judgment orestimation based upon patient demographics. In one example, the oxygenuptake rate can be estimated using the patient's weight and the Brodyequation. The DSP 36 calculates the oxygen uptake rate from the inspiredoxygen concentration and the expired oxygen concentration using equation(2).

In still further embodiments, the oxygen uptake rate can be crudelyestimated based upon safe population base values. This can be assumed tobe the easiest, but the least accurate as there is a wide range ofpopulation need (e.g. 300 milliliters per minute or more for an adultwith a fever, or less than 20 milliliters per minute in a neonate). Therelationship of the fresh gas oxygen concentration to the fresh gas flowrate and the fresh gas oxygen flow rate are shown below in equations (3)and (4):

$\begin{matrix}{{FFGO}_{2} = \frac{{FGO}_{2}F}{FGF}} & (5) \\{{FGF} = \frac{{FGO}_{2}F}{{FFGO}_{2}}} & (6)\end{matrix}$

Delivery of fresh gas oxygen and a balance gas requires two parametersto control the fresh gas manifold 34. The complementary pairs can bechosen from the concentration of one of the gases and the total freshgas flow, or the flow of each individual gas. Two commonly usedcombinations of these complementary setting pairs are FFGO₂ and FGF, orFGO₂F and a balance gas flow rate (BGF). In one embodiment of thedisclosed hypoxic guard, the FGO₂F or the FGF is derived using equations1 and 2 using values of FiO2 or FeO2 that correspond to the desiredprotection for the delivery of the safe non-hypoxic thresholdconcentration, and an estimated or assumed oxygen uptake (VO2). Atypical safe threshold value for either FiO₂ or FeO₂ is 25%. It can beseen from equation 1 that the relationships of the variables areindependent of the ventilator parameters. As such, FeO₂ works well toestablish the safe non-hypoxic delivery to the patient. Since FiO₂ isconventionally measured to alarm on hypoxic delivery, the safe FiO₂goals are familiar to operating clinicians and technicians, thus, it isuseful in embodiments to use the FiO₂ as a safe non-hypoxic gasconcentration threshold for gases delivered to the patient. Dependingupon the combination of inputs being made by the clinician in the inputdevice 54 (e.g. FGF and. FFGO₂, or FGO₂F and BGF) the DSP 36 calculatesthe threshold value of the parameter being set using the current valueof the complementary parameter. The safe input threshold values are theminimum oxygen concentration, the minimum oxygen flow, the minimum freshgas flow rate, or the maximum balance gas flow rate.

Therefore, if the clinician attempts to enter a value for the fresh gasflow rate (FGF), then the DSP 36 calculates the minimum fresh gas flowrate using equations 1 and 2 using safe non-hypoxic FiO2 or FeO2threshold concentration, patient oxygen uptake (VO2), and the currentset value for the fresh gas oxygen concentration (FFGO₂). If the valueof the fresh gas flow rate being set is higher than the minimum freshgas flow rate, the DSP 36 will accept the setting being made, thusensuring that the gas manifold 34 will be controlled to provide adequateoxygen to the patient 12. If the clinician attempts to input a value forthe fresh gas oxygen concentration (FFGO₂), the DSP 36 calculates aminimum fresh gas oxygen concentration (FFGO₂) using equations 1 and 2using safe non-hypoxic FiO2 or FeO2 threshold concentration, patientoxygen uptake (VO2), and the current set value for the fresh gas flowrate that must be provided by the fresh gas manifold 34 to provideadequate oxygen in the inspired gases 16 to the patient 12. Settingsabove the minimum fresh gas oxygen concentration are values that the DSP36 will accept as safe non-hypoxic values to control the fresh gasmanifold 34. As a minimum provision, the clinician should be alerted toa hypoxic setting is attempted.

The relationship between fresh gas flow rate (FGF), balance gas flowrate (BGF), fresh gas oxygen flow rate (FGO₂F), and anesthetic agentvapor flow rate (AVF) can be determined by the following equation:FGF=BGF+FGO₂F+AVF  (7)

Additionally, combining equation (5) with equations (1) through (4), DSP36 can calculate the safe non-hypoxic range of FGO₂F, BGF, and AVFsettings to control the gas manifold 34. In this complementary pair offresh gas manifold settings, if the clinician attempts to input asetting value for the BGF, the DSP 36 calculates, using equations 1through 5, values of safe non-hypoxic FiO2 or FeO2 thresholdconcentration, patient oxygen uptake (VO2), and the current set valuefor the FGO₂F, a maximum BGF that must not be exceeded to ensure thatthe fresh gas manifold 34 will be controlled to provide adequate oxygento the patient 12. Only values that are below the maximum BGF will beaccepted by the DSP 36 to control the delivery of gases from the freshgas manifold 34. Settings above the minimum fresh gas oxygenconcentration are values that the DSP 36 will accept as safe non-hypoxicvalues to control the fresh gas manifold 34. In one embodiment, theclinician is alerted when a hypoxic BGF setting is attempted. In afurther embodiment, the DSP 34 uses a similar process when the clinicianattempts to set the FGO₂F to ensure that only safe non-hypoxic FGO₂Fsettings above the minimum FGO₂F is being set or (as a minimumrequirement) alert the clinician of a hypoxic FGO₂F setting isattempted.

While the above examples have been for single pairs of fresh gasmanifold parameters, it is to be recognized that in alternativeembodiments, the clinician may input or otherwise establish a pluralityof fresh gas manifold parameters. In this case, the DSP 36 wouldcalculate the required minimum or maximum values for the additionalparameters with respect to the input values at the equations asdescribed above. The addition of anesthetic vapor into the fresh gasline 46 by the anesthetic vaporizer 32 can be similarly treated as anadditional parameter.

In a still further embodiment of the respiratory support system 10, agraphical display 66 is communicatively connected to the DSP 36. Thegraphical display 66 is operated by the DSP 36 in order to presentinformation regarding the operation and settings of the respiratorysupport system 10. In an embodiment, when the clinician enters aventilator parameter value into the input device 54, the DSP 36determines if the modified ventilator parameter value will result in thedelivery of a hypoxic gas mixture to the patient if the remaining freshgas manifold or ventilator parameter settings remain unchanged. Thedetermination is made by comparing a predicted delivered oxygen gasconcentration to a minimal patient oxygen requirement threshold. If theDSP 36 determines that such a change to the ventilator parameter wouldresult in a hypoxic gas mixture delivered to the patient, then the DSP36 can reject the clinician's entry and produce an alarm, including avisual or audible alarm with the graphical display 66. In an embodiment,the graphical display 66 further presents a prompt or option to theclinician in order to override the alarm, such that the clinician mayselectively and deliberately change the ventilator parameter value,despite the warning that such a change on its own will result in thedelivery of a hypoxic gas to the patient.

FIG. 2 is a flow chart that depicts an embodiment of a method ofpreventing the delivery of hypoxic gases to a patient. It is to beunderstood that embodiments of the method as disclosed herein withrespect to FIG. 2 need not require each of the steps as detailed herein,nor specifically in the order described herein. Additionally,embodiments of the method may be performed solely through the use of acomputer that executes computer readable code stored on a computerreadable medium. The technical effect of such embodiments would be toprevent the delivery of hypoxic gases to a patient. Other embodiments ofthe method may be performed with a respiratory support system such as,but not limited to that depicted and disclosed in FIG. 1.

The method 100 starts with respiratory support being provided to thepatient at 102. The respiratory support provided at 102 can either befull mechanical ventilation or can be spontaneous breathing assistance.Additionally, the respiratory support may include the delivery ofanesthetic agent or other gaseous or nebulized drugs entrained in themedical gas delivered to the patient.

At 104, a ventilation parameter change is received by a digital signalprocessor (DSP). In one embodiment, the ventilation parameter valuechange is received by a clinician entering a change in the ventilationparameter value through an input device. In an alternative embodiment,the ventilation parameter value change is received from an automatedfresh gas manifold control system, or as part of a plurality ofventilation parameter value changes. In embodiments, the ventilationparameter value changed by the clinician includes, but is not limitedto, FGF, FFGO₂, MV, FGO₂F, and BGF. A person of ordinary skill in theart would recognize alternative ventilation parameter values that mayadditionally or alternatively be used in embodiments of the systems andmethods as disclosed herein.

The DSP digital signal processor calculates a prediction of the oxygenconcentration that will be delivered to the patient at 106 if theventilation parameter value change is made. As described above withrespect to FIG. 1, the calculated delivered oxygen concentrationprediction can be calculated using one or more equations as disclosedabove that relate the concentration and flow rate of expired gases fromthe patient and the fresh gas supply to the concentration and flow rateof inspiratory gases provided to the patient.

At 108, the predicted oxygen concentration is compared to apredetermined threshold that is representative of a minimal oxygenmetabolic need of the patient. The patient's metabolic oxygen need, oroxygen uptake can be supplied to the digital signal processor in avariety of ways. In some embodiments, the patient oxygen uptake ismeasured directly or calculated from inspired gas and expired gas oxygenconcentration values. Alternatively, a population or other demographicestimate of oxygen need can be used as a substitute for a measured orderived value that would be more patient specific.

If the predicted delivered oxygen concentration is above thepredetermined minimum of oxygen concentration threshold, then at 110,the digital signal processor accepts the ventilation parameter value tothe value that had been received at 104. The method 100 continues backto 102 where respiratory support is provided to the patient with the newand updated ventilation parameter value.

If the predicted delivered oxygen concentration does not meet thepredetermined minimum oxygen concentration threshold, then at 112, thedigital signal processor rejects the ventilation parameter value changethat was received at 104.

In one embodiment, after the ventilation parameter value change has beenrejected at 112, the digital signal processor operates a graphicaldisplay or other output device to produce an alarm at 114. The alarmproduced at 114 serves to notify the clinician that a hypoxic gasmixture would be delivered to the patient if the ventilation parametervalue from 104 were accepted. The alarm may further include a prompt forthe clinician to override the alarm. At 116, a signal to override thealarm is received. A clinician may enter such an override signal throughan input device. This feature or control may be desired by a clinicianin an event wherein the clinician has particular knowledge of thespecific respiratory therapy or condition of the patient. Therefore, theclinician specific knowledge can be implemented and the clinician canoverride the alarm to deliver the respiratory support according to theventilation parameter value input by the clinician. After the alarm isoverridden at 116, the ventilation parameter is accepted at 110 andrespiratory support is provided to the patient according to the newventilation parameter value.

Alternatively, after the digital signal processor rejects theventilation parameter change at 112, the DSP calculates a new value forat least one additional parameter at 118. The DSP calculates at leastone additional parameter value change that would be required to acceptthe input ventilation parameter value change and still prevent thedelivery of a hypoxic gas mixture to the patient. In one embodiment,this is performed through an iterative process wherein a new value forat least one additional parameter is used to calculate a new deliveredoxygen concentration prediction at 102. The new delivered oxygenconcentration prediction is compared to the predetermined threshold at122. If the new delivered oxygen concentration prediction is still belowthe threshold, then a new value for the at least one additionalparameter is calculated at 118 and the loop repeats. Non-limitingexamples of additional parameter values that are changed include thefresh gas oxygen concentration (FFGO₂) and the fresh gas flow rate(FGF).

If the new delivered oxygen concentration prediction based upon thereceived ventilation parameter value and the at least one additionalparameter value is above the predetermined threshold at 122, than at 124the DSP accepts the ventilation parameter value and the at least oneadditional parameter value. Respiratory support is then provided to thepatient using the new ventilation parameter value and at least oneadditional parameter value.

The inventors have observed that some clinicians have had difficulty inadopting ventilation control and anesthesia delivery techniques tomaximize the benefits gained from low-flow respiratory support systems.Exemplarily, a clinician may inefficiently use a low-flow respirationsupport system operating at a fresh gas flow of two or three liters perminute when the low-flow respiratory support system has the capabilityto safely provide respiratory support at flows of 500 milliliters perminute or less. The methods and systems as disclosed herein can furtherbe leveraged to assist clinicians in more efficiently using thecapability of the low-flow respiratory support system by providingadditional information and feedback so that the clinician canself-correct.

FIG. 3 depicts an example of a screen shot of an embodiment of agraphical user interface as may be presented on graphical display 66(FIG. 1). The graphical user interface (GUI) 76 presents a variety ofinformation related to the provision of ventilatory assistance and thedelivery of anesthetic agent to the patient. The GUI 76 includes region78, which may be a separate graphical user interface, or a subordinategraphical interface to the larger GUI 76. Region 78 includes a totalflow graph 80. The total flow graph 80 graphically presents the flow ofconstituent gases exiting the mixer 70 of the respiratory support system10 (FIG. 1). In the embodiment of the total flow graph 80 as depicted inFIG. 3, the top bar 82 represents the total flow rate (exemplarilyexpressed as a rate in liters per minute) of fresh gas out of the mixer.In an embodiment, the top bar 82 graphically depicts a bobbin or floatas traditionally used in mechanical meters of medical gas flow.

Below the top bar 82 are two regions: a balance gas region 84 and anoxygen region 86. Both of these regions respectively graphical indicatethe flow rates of these two components to the total fresh gas flow. Aswill be explained in further detail herein, fresh gas flow out of themixer includes an oxygen component comprising the oxygen from airsupplied to the mixer as well as supplemental oxygen provided from anoxygen source. The remainder of the total fresh gas flow can beconsidered a balance gas making up the remainder of the total flow graph80. In embodiments, this also includes the component attributable toanesthetic agent vapor. By superimposing the oxygen flow rate as theoxygen region 86 over the flow rate of all the fresh gas, the total flowgraph 80 also represents the concentration of oxygen in the fresh gassupplied by the mixer.

The graphical user interface 76 can further include an additional regionor subordinate graphical user interface 88 that includes control buttonsor icons such as to select and confirm settings for the fresh gassupplied from the mixer. Exemplarily, an oxygen concentration button 90and a total flow button 92 are depicted in the region 88. By entering achange to the oxygen concentration or to the total flow rate with therespective buttons 90, 92, the total flow, and resulting oxygen flowcomponent represented in the total flow graph 80 will changeaccordingly, as explained above with respect to equation 5.

The region 78 further includes a delivered oxygen reference 94. Asexplained above and detailed further herein, an oxygen concentration asdelivered to the patient can be calculated from various measurements andinputs in the respiratory support system, particularly by application ofequation 1. In a further manner of leveraging this technology, a valuefor an oxygen flow rate that achieves a predetermined referenced oxygenconcentration as delivered to the patient can be calculated andsuperimposed as the delivered oxygen reference 94 on the total flowgraph 80. Therefore, the delivered oxygen reference 94 provides furtherguidance to a clinician such that a reference oxygen concentration asdelivered is presented based upon the oxygen flow rate out of the mixerthat achieves the delivery of that oxygen concentration to the patientat the current total fresh gas flow rate out of the mixer.

The region 78 further includes an anesthetic agent flow graph 96. Theanesthetic agent flow is determined as disclosed in further detailsherein, and is presented, exemplarily at a rate of milliliters (mL) perhour in the anesthetic agent flow graph 96. As further described herein,the anesthetic agent flow rate can be converted into an anesthetic agentcost estimate 98 that is presented in the region 78. In the exampleprovided in FIG. 3, the anesthetic agent cost estimate is furtherpresented as a rate, exemplarily dollars per hour. Details of thegraphical user interface 76 and the determination and presentation ofthe values described above are explained in further detail herein,particularly with respect to the flow chart of FIG. 4.

FIG. 4 is a flow chart that depicts an embodiment of a method ofcontrolling the delivery of medical gases 200. The method 200 depictedin FIG. 4 includes similarities to portions of the flow chart depictedin FIG. 2, and it is understood that in embodiments, both methods 100(FIG. 2) and 200 (FIG. 4) may be conducted concurrently. At 202, similarto step 104 in FIG. 2, a ventilation parameter value change is received.As mentioned above, the change to the ventilation parameter value can beany of a variety of changes to a variety of ventilation parametervalues, including, but not limited to FGF, FFGO₂, MV, FGO₂F, and BGF. Inother embodiment, the received ventilation parameter change can includeanesthetic agent vapor flow rate (AVF). In still further embodiments,the received ventilation parameter change can be an updated measuredvalue of any of the above parameters, or another ventilation parametervalue as would be recognized by one of ordinary skill during the courseof operation of the respiratory support system.

After the method 200 is initiated with the receipt of a ventilationparameter value change at 202, then one or more of 204, 206, and 208 areperformed in order to calculate one or more values needed to carry outthe method 200. It is to be understood that if the received ventilationparameter value change at 202 is one of the values calculated at 204,206, or 208, such calculations result in the received ventilationparameter value from 202.

At 204, the predicted delivered oxygen concentration to the patient iscalculated such as explained above with respect to FIGS. 1 and 2 andparticularly using equation (1). At 206 a fresh oxygen flow rate intothe breathing circuit is calculated. At 208 the total fresh gas flowrate into the breathing circuit is calculated. The relationship betweenthe fresh oxygen flow and the total fresh gas flow is discussed abovewith respect to equation 7.

At 210 the total fresh gas flow from 208 and the fresh oxygen flow from206 are presented on a graphical display. An exemplary embodiment ofsuch a presentation is depicted in FIG. 3. As described above withrespect to FIG. 3, the fresh oxygen flow can be presented assuperimposed on top of the total fresh gas flow such that theconcentration of oxygen in the fresh gas flow into the breathing circuitis further also graphically represented. It can also be recognized thatin alternative embodiments, the concentration of oxygen in the fresh gasflow is calculated at 206, rather than the flow rate of oxygen in thefresh gas flow as these values are mathematically related and indicativeof the same system properties of the fresh gas flow entering thebreathing circuit.

At 212 a reference oxygen flow into the breathing circuit required toachieve a predetermined oxygen concentration delivery to the patient iscalculated from at least the total fresh gas flow from 208. This is afurther leveraging of the equations as explained above wherein at 212the reference oxygen flow to achieve the delivery of a predeterminedoxygen concentration is calculated in the same manner as described abovewith respect to equations 1-2; however, in some embodiments of 212, thepredetermined oxygen concentration need not be a minimum oxygenconcentration (nominally 21%), but may be another referenceconcentration value such as to assist a clinician in selecting theventilation parameter value, or a high reference oxygen concentrationsuch as to prevent oxygen toxicity. Non-limiting examples of thepredetermined oxygen concentration delivered to the patient may include25% oxygen or 30% oxygen although these examples are not intended to belimiting on the predetermined oxygen concentrations used at 212. If thereference is a high reference to warn of oxygen toxicity, thepredetermined concentration may exemplarily be 90%. As noted above,general anesthesia practice would generally recommend that thepredetermined oxygen concentration not be below the 21% oxygen found inair, below which would constitute a hypoxic mixture and, as such, may bedesired to be avoided.

After the reference oxygen flow is calculated at 212 an indication ofthe calculated reference oxygen flow into the breathing circuit requiredto deliver the predetermined oxygen concentration to the patient ispresented on the graphical display 214. In one embodiment as depicted inFIG. 3, the indication of the calculated reference oxygen flow ispresented as a reference tab on the total flow graph that is labeledwith the predetermined oxygen concentration delivered to the patient,but represents the fresh oxygen flow rate into the breathing circuitthat is required to achieve the delivery that predetermined oxygenconcentration to the patient.

In mechanically actuated vaporizers, neither the anestheticconcentration setting nor the vapor flow rate are electronicallymeasured but can be calculated from at least one variable from theventilator and the respiratory gas monitor connected to the patient. At216 an anesthetic vapor flow rate (AVF) is calculated. The calculationof the anesthetic vapor flow rate is calculated in a similar manner tothat described above with respect to the calculation of the oxygen asdelivered to the patient. The anesthetic vapor flow rate is calculatedbased upon one of two equations depending upon whether the expiredminute volume (MVe) is greater than or less than the total fresh gasflow provided by the mixer (FGF). If the MVe is greater than or equal toFGF, then the anesthetic agent vapor flow rate is calculated by:

$\begin{matrix}{{{AVF}\mspace{14mu}\left( {{mL}\text{/}\min} \right)} = \frac{{{FiA} \cdot {MVi}} - {{FeA} \cdot {MVe}} - {FGF}}{1 - {FeA}}} & (8)\end{matrix}$

If the MVe is less than the FGF, then the anesthetic flow rate iscalculated by:

$\begin{matrix}{{{AVF}\mspace{14mu}\left( {{mL}\text{/}\min} \right)} = \frac{{FiA} \cdot {FGF}}{1 - {FiA}}} & (9)\end{matrix}$

It is to be noted that in the above equations 6 and 7, if the MVe isgreater than the inspired minute volume (MVi), the MVe value is setequal to the MVi value. The above equations 8 and 9 use the fraction ofagent inspired (FiA) and fraction of agent expired (FeA) in calculatingthe rate per minute at which the anesthetic agent vapor is delivered. Itis to be noted that if AVF is desired to be represented at analternative rate, exemplarily millimeters per hour as depicted in FIG.3, then the above equations 8 and 9 would include the appropriateconversation, exemplarily multiplying by 60 minutes per hour. The FiAand FeA values used in equations 8 and 9 may be calculated or obtainedfrom the vaporizer system that measures the amount of anesthetic agentthat is being metered out through the vaporizer. In a still furtherembodiment, the anesthetic vapor flow rate may be measured directly froma gas analyzer disposed in the breathing circuit.

After the anesthetic vapor flow rate is calculated at 216, the cost ofthe anesthetic agent cost consumed to supply the anesthetic vapor flowis calculated at 218. In order to first estimate the cost of theanesthetic agent being used, the anesthetic vapor flow rate (AVF) mustbe converted to a liquid agent flow rate. This is calculated usingequation 10:

$\begin{matrix}{{ALF} = \frac{AVF}{{CONVERSION}\mspace{14mu}{FACTOR}}} & (10)\end{matrix}$

In equation 8 the liquid anesthetic flow (ALF) is calculated by dividingthe anesthetic vapor flow by a conversion factor. Exemplary conversionfactors are provided in the following table.

Anesthetic Agent Conversion Factor desflurane 208 enflurane 198halothane 227 isoflurane 196 sevoflurane 181

As shown in the Table above, the conversation factors are specific tothe anesthetic agent and convert the vapor flow rate of the anestheticagent into a liquid flow rate as anesthetic agents are typically soldand priced in their liquid form. The hospital or institution may enter acurrent cost associated with each anesthetic agent that may be used bythe respiratory support system and the estimated anesthetic agent costis calculated at 218 by multiplying the anesthetic agent cost by theliquid anesthetic agent flow rate to arrive at a cost per unit of timethat the anesthetic agent is provided at that rate. It is understoodthat the estimated anesthetic agent cost can be localized to the countryor region in which the respiratory support system is used by changingthe currency or cost per volume references in the estimated anestheticagent cost.

At 220, the anesthetic vapor flow rate and anesthetic agent cost areboth presented on the graphical display. As referenced above, this isexemplarily depicted in FIG. 3 by the anesthetic agent flow graph 96 andthe anesthetic agent cost estimate 98. It is to be noted that theanesthetic agent cost estimate is presented in a rate of dollars perhour, but as disclosed above, this can be localized.

In some embodiments of the method 200, at 222 the presentations ofinformation reflecting the ventilatory and anesthesia support to beprovided to the patient particularly from 210, 214, and 220 within themethod provide the clinician with additional insight and guidance as tothe respiratory support to be provided and the user or clinician isprompted to confirm the entered ventilation parameter change after beinginformed of the effects of that change by the presentation as hereinjust described. It is to be noted that in other embodiments, theventilation parameter value change may be implemented by the respiratorsupport system as soon as the parameter value change is received at 202,in such embodiments, the method 200 may be carried out in real time,continuously representing the actual conditions of the respiratorysupport provided to the patient.

As noted above, an anesthesiologist or other clinician may use greaterflow rates of fresh gas and anesthetic agent than are required by modernlow flow respiratory systems. Part of this may stem from caution, or maybe a result of clinicians being inexperienced with extent of the flowcapabilities of low flow ventilatory support systems. By visuallypresenting the clinician with this information, the clinician can seethat equivalent respiratory support can be provided at lower fresh gasand anesthetic agent consumption rates. Such changes in operation canresult in reduced cost of ownership and wasted medical gas andanesthetic agent.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The patentable scope of the inventionis defined by the claims, and may include other examples that occur tothose skilled in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral languages of the claims.

What is claimed is:
 1. A system for controlling the delivery of medicalgases during respiratory support of a patient, the system comprising: abreathing circuit including a patient connection adapted to deliverrespiratory support to the patient; a mixer pneumatically connected tothe breathing circuit, the mixer configured to provide at least oxygenand a balance gas to the breathing circuit; a mechanical ventilatorpneumatically connected to the breathing circuit, the mechanicalventilator configured to create fluid pressure waveforms within thebreathing circuit to provide respiratory support to the patient; adigital signal processor communicatively connected to the mixer and themechanical ventilator, the digital signal processor receives at leastone ventilation parameter value change, calculates a fresh oxygen flowrate into the breathing circuit and a total fresh gas flow rate into thebreathing circuit with the at least one ventilation parameter value, andcalculates a reference oxygen flow rate representative of apredetermined oxygen concentration delivered to the patient with atleast the total fresh gas flow rate; and a graphical displaycommunicatively connected to the digital signal processor, wherein thegraphical display presents the calculated fresh oxygen flow rate, thetotal fresh gas flow rate, and the reference oxygen flow rate.
 2. Thesystem of claim 1, wherein the predetermined oxygen concentration isestablished by a clinician.
 3. The system of claim 1, furthercomprising: a vaporizer pneumatically connected to the breathingcircuit; wherein the digital signal processor calculates an anestheticagent consumption rate at least in part from the total fresh gas flowrate, calculates an estimated cost from the anesthetic agent consumptionrate, and operates the graphical display to present the estimated cost.4. The system of claim 3, wherein the digital signal processorcalculates an anesthetic agent vapor flow rate in the total gas flowrate.
 5. The system of claim 4, wherein the digital signal processorconverts the anesthetic agent vapor flow rate into an anesthetic liquidflow rate and uses the anesthetic agent liquid flow rate as theanesthetic agent consumption rate.
 6. The system of claim 1, wherein thegraphical display presents the fresh oxygen flow rate and total freshgas flow rate in a single graphical user interface, the single graphicaluser interface further presents the reference oxygen flow rate as areference tab on the single graphical user interface.
 7. The system ofclaim 1, wherein the predetermined oxygen concentration is apredetermined hypoxic concentration threshold.
 8. The system of claim 1,further comprising: a respiratory gas monitor pneumatically connected tothe breathing circuit, the respiratory gas monitor produces signalsindicative of an oxygen concentration inhaled and exhaled by thepatient.
 9. The system of claim 8, wherein the digital signal processorcalculates the predetermined minimum oxygen requirement threshold usingat least one of the signals indicative of oxygen concentration inhaledand exhaled by the patient.
 10. The system of claim 8, whereinrespiratory gas monitor further produces signals indicative of ananesthetic agent vapor concentration inhaled and exhaled by the patient,wherein the digital signal processor uses the signals received from therespiratory gas monitor to calculate an estimated cost of consumedanesthetic agent.
 11. A low-flow ventilation system provides repeatingbreaths of respiratory support to a patient, the system comprising: abreathing circuit including a patient connection adapted to deliver therespiratory support to the patient; an input device operable by aclinician to input at least one ventilation parameter value; a mixerpneumatically connected to the breathing circuit, the mixer configuredto provide at least oxygen and a balance gas to the breathing circuit; avaporizer pneumatically connected to the breathing circuit; a digitalsignal processor communicatively connected to the input device, thedigital signal processor receives the at least one ventilation parametervalue, calculates a fresh oxygen flow rate into the breathing circuit,calculates total fresh gas flow rate into the breathing circuit,calculates a predicted oxygen concentration delivered to the patient,and compares the predicted oxygen concentration to a predeterminedminimum oxygen requirement threshold of the patient; a graphical displaycommunicatively connected to the digital signal processor, the graphicaldisplay presents the calculated fresh oxygen flow rate and total freshgas flow rate; wherein if the predicted oxygen concentration is abovethe predetermined minimum oxygen requirement threshold, the digitalsignal processor accepts the at least one ventilation parameter value.12. The system of claim 11, wherein if the predicted oxygenconcentration delivered to the patient is below the predeterminedminimum oxygen requirement threshold, the digital signal processorcalculates at least one additional parameter value such that thepredicted oxygen concentration is above the predetermined minimum oxygenrequirement threshold, and the digital signal processor accepts the atleast one ventilation parameter value and at least one additionalparameter value.
 13. The system of claim 11, wherein the digital signalprocessor further calculates a reference oxygen flow rate required toachieve a delivery of a predetermined oxygen concentration to thepatient and the graphical display presents a graphical indication of thecalculated reference oxygen flow rate in relation to the calculatedfresh oxygen flow rate.
 14. The system of claim 13, wherein the digitalsignal processor further calculates a predicted anesthetic vapor flowrate at least in part from the total fresh gas flow rate, the digitalsignal processor further converts the predicted anesthetic vapor flowrate to an anesthetic agent consumption cost and the graphical displaypresents the anesthetic agent consumption cost.
 15. The system of claim14, further comprising: a respiratory gas monitor pneumaticallyconnected to the breathing circuit, the respiratory gas monitor producessignals indicative of an anesthetic agent concentration inhaled andexhaled by the patient; wherein the digital signal processor calculatesthe anesthetic vapor flow rate from at least the indicated inhaled andexhaled anesthetic agent concentrations and the total fresh gas flowrate.
 16. A method of controlling the delivery of medical gases to apatient receiving ventilatory support from a low-flow mechanicalventilator, the method comprising: providing ventilatory support to thepatient through a breathing circuit pneumatically connected to amechanical ventilator in a low-flow configuration; receiving, with amixer pneumatically connected to the breathing circuit, a flow ofmedical gas comprising oxygen; receiving, with a digital signalprocessor, a ventilation parameter value change; calculating, with thedigital signal processor, a total fresh gas flow rate into the breathingcircuit from the received ventilation parameter value change;calculating, with the digital signal processor, a fresh oxygen flow rateinto the breathing circuit from the received ventilation parameter valuechange; calculating, with the digital signal processor, a referenceoxygen flow rate into the breathing circuit that achieves apredetermined oxygen concentration as delivered to the patient basedupon the total fresh gas flow rate; and presenting the total fresh gasflow rate, the fresh oxygen flow rate, and the reference oxygen flowrate on a graphical display.
 17. The method of claim 16, furthercomprising: providing a graphical indication of the calculated referenceoxygen flow rate on a single graph with the fresh oxygen flow rate andthe total fresh gas flow rate on the graphical display.
 18. The methodof claim 16, further comprising: calculating a predicted oxygenconcentration delivered to the patient with the digital signal processorbased upon the ventilation parameter value change; comparing thepredicted oxygen concentration to the predetermined oxygen concentrationwith the digital signal processor; accepting, with the digital signalprocessor, the ventilation parameter value if the predicted oxygenconcentration is above the predetermined oxygen concentration;rejecting, with the digital signal processor, the ventilation parametervalue if the predicted oxygen concentration is below the predeterminedoxygen concentration.
 19. The method of claim 18, wherein predeterminedoxygen concentration is a predetermined hypoxic concentration threshold.20. The method of claim 16, further comprising: determining an amount ofanesthetic agent consumed by operation of a vaporizer pneumaticallyconnected to the breathing circuit at the total fresh gas flow rate;calculating an equivalent cost for the consumed anesthetic agent withthe digital signal processor; and presenting the calculated equivalentcost on the graphical display.